1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to an asymmetric wireless communication system applied to an equally distributed network.
2. Description of the Related Art
Conventionally, in a network including a wireless base station and a plurality of wireless terminal devices, an asymmetric wireless access system in which a space division multiple access (SDMA) scheme is used for a downlink line which is large in communication volume from a base station to a terminal while a time division multiple access (TDMA) scheme is used for a uplink line which is relatively small in communication volume from a terminal to a base station is under review. Japanese Patent No. 7926669 discloses a wireless communication system employing such an asymmetric wireless access system.
Here, the principle of the SDMA scheme is briefly described based on Japanese Patent No. 4435039, FIG. 12 is a functional block diagram of a transmitting device for spatial multiplexing transmission disclosed in Japanese Patent No. 4435039. As illustrated in FIG. 12, a transmitting device 700 for spatial multiplexing transmission includes a distributing unit 710 that distributes a transmission signal sequence input from a transmission signal input terminal; a plurality of modulating units 720(1) to 720(n); a plurality of directivity forming units 740(1) to 740(n), a plurality of synthesizing units 760(1) to 760(n), and a plurality of antenna elements 780(1) to 780(n). The modulating units 720(1) to 720(n) perform previously set modulation on signals distributed by the distributing unit 710 and output modulation signals to the directivity forming units 740(1) to 740(n). The directivity forming units 740(1) to 740(n) generate modulation signals which are different in amplitude and phase for each antenna by multiplying the modulation signals from the modulating units 720(1) to 720(n) by weight coefficients specific to the respective antennas. The generated modulation signals are output to the antenna elements 780(1) to (780(n) via the synthesizing units 760(1) to 760(n) provided for the respective antenna elements. In this way, the modulation signals having different amplitudes and phases transmitted from the respective antennas are spatially multiplexed, and thus modulation signals having directivities are generated. In the transmitting device 700 for spatial multiplexing transmission, the modulation signals (the modulation signals having different amplitudes and phases for the respective antennas) that are output from the directivity forming units 740(1) to 740(n) are input to and synthesized (added up) by each of the synthesizing units 760(1) to 760(n) provided for the respective antenna elements and then transmitted from the antenna elements 780(1) to 780(n). As a result, the modulation signals having different directivities are generated as many as the number of the directivity forming units 740(1) to 740(n), so that space division multiplexing transmission is realized.
Meanwhile, an equally distributed network in which terminals communicate directly with each other without a base station has attracted attention recently. Here, when the SDMA scheme is applied to the equally distributed network having no base station, in a system including N terminals, if all terminals transmit signals of (N−1) paths to the other terminals at the same time, a huge number of space paths of N×(N−1) are generated within the system, influence of an interference wave is problematic. In addition, a receiving side terminal needs to process signals from (N−1) other terminals at the same time, and thus there is a problem n that a load thereof becomes vast. Further, when such communication is performed at a common frequency, since a transmitting terminal and a receiving terminal need to be separated, complicated control is necessary. In addition, in the case of a mobile terminal, since the position of the terminal is not fixed, how to control directivity of a transmitting signal toward a moving terminal becomes problematic.